The duration of pregnancy in dogs, elephants and cats is 63, 624 and 63 days respectively. Placenta plays an important role in pregnancy.

Placenta:

Placenta is the intimate connection between the foetus and uterine wall of the mother to exchange the materials. The outer surface of the chorion in humans develops a number of finger like projections, known as chorionic villi, which grow into the tissue of the uterus.

These villi, penetrate the tissues of the uterine wall in which they are embedded, make up the organ known as the placenta by means of which the developing embryo obtains nutri­ents and oxygen and gets rid of carbon dioxide and metabolic wastes.

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Because the chorion takes part in the formation of placenta, the human placenta is called the chorionic placenta. It consists of foetal part, the chorion and a ma­ternal part the decidua basalis.

The foetal part of placenta grows to invade the uterine mucosa with its chorionic villi. The degree of inti­macy is so strong that the blood vessels of the chorionic villi are bathed in the mother’s blood. This is due to erosion of the uterine mucosa, including its epithelium,
connective tissue and the endothelial lining.

This type of placenta which is based on the intimacy between foetal and maternal parts of the placenta, is referred to as haemochorial placenta. The placenta is connected to embryo through an umbilical cord which helps in the transport of substances to and from the embryo. On the basis of the distribution of villi on chorion, human placenta is called metadiscoidal placenta.

The placenta performs the following functions:

(i) Nutrition:

All the nutritive elements from the maternal blood pass into the foetus through the placenta,

(ii) Respiration:

Oxygen passes from the maternal blood to the foetal blood through the placenta, and carbon dioxide passes in the reverse direction,

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(iii) Excretion:

The foetal excretory products diffuse into the maternal blood through placenta and are excreted by the mother,

(iv) Storage:

Placenta stores glycogen, fat, etc.

(v) As a Barrier:

Placenta serves as an efficient barrier and allows those materials to pass into the foetal blood that are necessary. Teratogens are certain agents (viruses or chemicals) or drugs that cause abnormal development in developing embryo/ foetus. The most well known synthetic teratogen drug is thalidomide. This drug causes multiple defects in the growing embryo,

The hCG stimulates and maintains the corpus luteum to secrete progesterone until the end of pregnancy. The hCS stimulates the growth of the mammary glands during pregnancy. Relaxin facilitates parturition (act of birth) by softening the connective tissue of the pubic symphysis.

In addition, the levels of hormones like oestrogens, progestogens, cortisol, prolactin, thyroxine, etc. are increased in the maternal blood during pregnancy. Increased production of these hormones is necessary for supporting the foetal growth, metabolic changes in the mother and maintenance of pregnancy.

Important Developmental Changes in the Human Embryo:

Time from Fertilization

Organs Formed

Week 1

Fertilisation cleavage starts about 24 hours after fertilization. Cleavage to form a blastocyst 4—5 days after fertilisation. More than 100 cells. Implantation 6-9 days after fertilisation.

Week 2

The three primary germ layers (ectoderm, endoderm and mesoderm) develop.

Week 3

Woman will not have a period. This may be the first sign that she is pregnant. Beginnings of the backbone. Neural tube deve­lops, the beginning of the brain and spinal cord (first oragans).

Parturition and Lactation:

Parturition (L. Parturio = to be in labour):

Meaning:

The duration of pregnancy in human beings is about 9 months ± 7 days which is called gestation period. Infact, the gestation period is the time from conception to birth. At the end of the pregnancy vigorous contraction of uterus causes delivery or expulsion of the foetus. This act of expelling the full term young one from the mother’s uterus at the end of gestation period is called parturition.

Process:

Process of parturition is induced by both nervous system and hormones se­creted by the endocrine glands of the mother. The signals for child birth (parturition) originate from the fully matured foetus and placenta which induce mild uterine contractions called foetal ejection reflex. This causes quick release of oxytocin from the maternal posterior lobe of pituitary gland. The amount of oxytocin is increased just before and during “labour pains” (pains of child birth).

Childbirth begins with a long series of involuntary contractions of the uterus experienced as labour pains. Oxytocin (birth hormone) promotes contraction of the uterine muscles. Relaxin increases the flexibility of the pubis symphysis and ligaments of the sacroiliac and sacrococcygeal joints and helps dilate the uterine cervix during labour pains. Both of these actions give relief to the body from the pain during delivery of the baby.

The hormone most recently found to be produced by the placenta is corticotro- pin-releasing hormone (CRH), which in non-pregnant women is secreted only by neuro­-secretory cells in the hypothalamus. CRH is now thought to be part of the “clock” that establishes the timing of birth.

Secretion of CRH by placenta increases enormously toward the end of pregnancy. Women who have higher levels of CRH earlier in pregnancy are more likely to deliver prematurely, whereas those who have low levels are more likely to deliver after their due date.

Stages:

Labour pains can be divided into three stages:

(i) Stage of dilation:

The time from the onset of labour pain to the complete dilation of the cervix is called the stage of dilation. This stage lasts 6-12 hours. During this stage, regular contractions of the uterus, usually rupturing of the amniotic sac and complete dilation of the cervix occur. The first result of labour pains is the opening of the cervix. The amniotic fluid (the “waters”) starts flowing out through the vagina.

(ii) Stage of expulsion:

The time from complete cervical dilation to delivery of the baby is the stage of expulsion. It lasts 10 minutes to several hours. The baby passes through the cervix and vagina and is ‘delivered’ or born.

(iii) Placental Stage:

The time after the delivery until the placenta or “afterbirth” is expelled by powerful uterine contraction is the placental stage. These contractions also constrict blood vessels that were torn during delivery thereby reducing the possibility of haemorrhage.

In about 28-35 days, the uterus returns fully to its non-pregnant state by reduction in size and restoration of endometrium of the uterus.

Lactation:

Meaning:

Production of milk in the mammary glands is called lactation.

Period:

The female’s mammary glands undergo differentiation during pregnancy and start producing milk towards the end of pregnancy and after the birth of the young one.

Role of Hormones:

At puberty in females mammary glands begin to develop under the influence of oestrogen and progesterone. Secretion and storage of milk generally begins after birth of young one, usually within 24 hours under the influence of hormone prolactin (PRL) secreted by anterior lobe of the pituitary gland. However, the ejection of milk is stimulated by the hormone oxytocin (ОТ) released from the posterior lobe of the pituitary gland.

Colostrum:

The first milk which comes from the mammary glands of the mother just after child birth, for 2 or 3 days is called the colostrum. This is yellowish fluid that contains Cells from the alveoli and rich in protein (lactalbumin and lactoprotein) but low in fat. Colostrum contains antibodies (IgA is the major immunoglobin in it) that provide passive immunity to the new born infant.

Composition of Milk:

Human milk consists of water and organic and inorganic sub­stances. Its main constituents are fat (fat droplets), casein (milk protein), lactose (milk sugar), mineral salts (sodium, calcium, potassium, phosphorous, etc.) and vitamins. Milk is poor in iron content. Vitamin С is present in very small quantity in milk.

The process of milk secretion is regulated by the nervous system. It is also influenced by the psychic state of the mother. The process of milk production is also influenced by hormones of the pituitary gland (already mentioned), the ovaries and other endocrine glands. A nursing woman secretes of 1 to 2 litres of milk per day.

Importance of Breast feeding:

Breast feeding during initial stage of infant growth is recommended by doctors for the healthy baby. Milk contains an inhibitory peptide. If the mammary glands are not fully emptied, the peptide accumulates and inhibits milk production. Breast feeding is also a means of birth control, but it is not reliable.

Developmental Disorders:

1. Amnionitis (amnion + Gr suffix – ids – inflammation):

Inflammation of amnion, usually resulting from premature rupture of the amnion and often associated with neonatal infection.

2. Abortion:

It is giving birth to an embryo or foetus prior to the stage of viability at about 20 weeks of gestation (foetus weighs less than 500 gm). It may occur from natural causes or induced.